1. Field of the Invention
The present invention relates to a double-faced fluorescent display tube comprising an envelope including luminescent display portions on the respective inner surfaces of front and back substrates, cathodes for emitting electrons and control electrodes, and a method of manufacturing the double-faced fluorescent display tube.
2. Description of the Related Art
A double-faced fluorescent display tube has been heretofore used as a display apparatus in various fields of audio equipment or the like. This double-faced fluorescent display tube comprises an envelope formed of facing front and back substrates and a side plate, the envelope including: luminescent display portions composed of a phosphor layer and anodes which the phosphor layer is deposited on; cathodes for emitting electrons; and control electrodes arranged between the luminescent display portions and the cathodes. The double-faced fluorescent display tube is known to be of types: one type of the tube in which two luminescent display portions are observed from one substrate side of the envelope; and another type of the tube in which the luminescent display portions are observed from the substrate sides.
FIG. 9 is a partially enlarged side sectional view showing an example of this type of conventional double-faced fluorescent display tube.
As shown in FIG. 9, the double-faced fluorescent display tube has an envelope 1. The envelope 1 is assembled like a box from back and front substrates 2 and 3 formed of a plate material such as transparent glass and a side plate which the back and front substrates are surrounded by. A wiring pattern 5 composed of an Al thin film is formed on the inner surface of the back substrate 2. A black color insulating layer 7 having a through-hole 6 is formed on the wiring pattern 5. An anode 8 is formed on the through-hole 6 of the insulating layer 7. The anode 8 is connected to the wiring pattern 5 through a conductive material 9 which the through-hole 6 is filled with. A phosphor layer 10 is formed on the anode 8, thereby constituting a back-side luminescent display portion 11. A mesh grid 12 is placed as the back-side control electrode over the back-side luminescent display portion 11 at a predetermined distance away therefrom.
On the other hand, stripe-like wiring patterns 13 composed of the Al thin film are formed on the inner surface of the front substrate 3 with a predetermined distance therebetween. A part of the wiring pattern 13 is connected to transparent anodes 14. A phosphor layer 15 is formed on the anodes 14, thereby constituting a front-side luminescent display portion 16. A mesh grid 17 is placed as the front-side control electrode under the front-side luminescent display portion 16 at a predetermined distance away therefrom. In the envelope 1, filamentary cathodes 18 for emitting the electrons are disposed between the mesh grids 12 and 17.
In the conventional double-faced fluorescent display tube constituted as described above, a space is provided around the wiring patterns 13 of the front substrate 3. It is thereby possible to observe, from the side of the front substrate 3, the luminescent display of both of the back-side luminescent display portion 11 on the back substrate 2 and the front-side luminescent display portion 16 on the inner surface of the front substrate 3. Thus, the wiring patterns 13 on the side of the front substrate 3 are exposed. This has a problem. That is, conductive contaminants or the like produced in a manufacturing step adhere to the space between the wiring patterns 13. As a result, a short-circuit occurs between wires, thus yielding insulation failure. Particularly, sparks generated by electrically welding the electrodes, a getter film, or the like move through the envelope 1 and adhere to the space between the wiring patterns 13. Due to this, the short-circuit takes place between the wiring patterns 13 and thus brings about the insulation failure, or the wiring patterns 13 are damaged and thus broken. Such a problem is prone to arise.
The present invention is therefore made in view of the above problem. An object of the present invention is to provide a double-faced fluorescent display tube capable of reducing the insulation failure and breaking of the wiring patterns caused due to the conductive contaminants or the like produced in the manufacturing step and capable of improving a luminous efficiency of the phosphor by reducing an ineffective current of the anode, and a method of manufacturing the double-faced fluorescent display tube.
In order to achieve the above object, according to a first aspect of the invention, there is provided a double-faced fluorescent display tube comprising: an envelope formed of two substrates facing each other whose outer periphery is sealed, in which the respective inner surfaces of the substrates have a luminescent display portion and a wiring pattern connected to the luminescent display portion, wherein an exposed portion of the wiring pattern is covered with a transparent insulating film.
According to a second aspect of the invention, there is provided a double-faced fluorescent display tube comprising: an envelope formed of back and front substrates facing each other and a frame-like side plate sealed between the back and front substrates, in which an inner surface of the back substrate has a wiring pattern, a black color insulating layer which the wiring pattern is covered with, an anode disposed on the insulating layer and connected to the wiring pattern through a through-hole, a phosphor layer disposed on the anode, and a control electrode disposed at a predetermined distance away from the phosphor layer; an inner surface of the front substrate has a wiring pattern, transparent anodes connected to the wiring pattern, a phosphor layer disposed on the anodes, and a control electrode disposed at a predetermined distance away from the phosphor layer; and cathodes are stretchedly disposed between the control electrodes, wherein an exposed portion of the wiring pattern on the front substrate is covered with a transparent insulating film.
According to a third aspect of the invention, there is provided a double-faced fluorescent display tube according to the second aspect, wherein the whole surface of the front substrate is covered with a transparent insulating film, except the luminescent display portion and an internal terminal.
According to a fourth aspect of the invention, there is provided a double-faced fluorescent display tube according to the first, second or third aspect, wherein the insulating film comprises SiO2 thin film.
According to a fifth aspect of the invention, there is provided a method of manufacturing a double-faced fluorescent display tube, the tube comprising an envelope formed of facing front and back substrates and a side plate, the envelope including: an inner surface of the front substrate having a plurality of luminescent display portions composed of a phosphor layer and anodes which the phosphor layer is deposited on, a wiring pattern which the plurality of anodes are connected to, and an internal terminal connected to the wiring pattern; cathodes for emitting electrons; and a control electrode arranged between the luminescent display portions and the cathodes, the method comprising the steps of: forming the anodes, the wiring pattern, the internal terminal and first register marks on the front substrate by using a first patterning member having a plurality of first register marks whose shape represents a misalignment tolerance which is a level that is allowed even when a second patterning member is misaligned; and forming a transparent insulating film on the front substrate by using a second patterning member having a plurality of second register marks corresponding to the first register marks, while the second register marks are matched to the first register marks formed on the front substrate within the misalignment tolerance.
According to a sixth aspect of the invention, the first register mark may be formed of a rectangular mark whose width is equal to the misalignment tolerance.
According to a seventh aspect of the invention, the second register mark may be formed of a mark having a blank whose dimension is identical with an outer dimension of the first register mark.
According to an eighth aspect of the invention, the second patterning member may be constituted so as to form the insulating film on the inner surface of the front substrate except the luminescent display portion and the internal terminal.